The corrosion of industrial piping and other components such as valves and pumps is a major problem in some industries. The oil industry, in particular, faces severely corrosive environments, with corrosive gasses and liquids such as H2S (hydrogen sulfide) at elevated temperatures and pressures. Additionally, these conditions form severe wear and erosion environments. One solution to these issues is to coat a lower grade base material with a high quality coating material having the desired high corrosion and wear-resistant properties. Typically, these types of properties will be found in metal, ceramic and particularly diamond-like carbon coatings.
Expensive specialty alloys, such as HASTELLOY and INCONEL (both of which are federally registered trademarks of Huntington Alloys Corporation), are commonly used for exhaust piping in the chemical processing industries. These alloys exhibit high temperature strength and corrosion resistance. Again, a less expensive base material can be used if a suitable surface coating is applied to the interior surface that is to be exposed to the corrosive environment.
Prior art coating methods for formation of diamond-like carbon include chemical vapor deposition (CVD), and physical vapor deposition (PVD) methods. Many of the desirable properties of DLC are determined by the amount of diamond bonding (sp3) compared to graphite bonding (sp2) of the carbon. By expanding the sp3/sp2 ratio it is possible to achieve many of the excellent properties of diamond such as high hardness, low coefficient of friction, low wear, high Young's modulus, chemical inertness, etc.
Composite coatings based on DLC have also been shown to have desirable properties. For example layered films using a material of low modulus followed by a material of high hardness such as WC/C has been shown to increase wear resistance. Similarly, a so called “nano-composite” can be used. A nano-composite is formed by mixing the materials instead of layering, so that nano-sized crystals of a very hard material (e.g. TiN) are embedded in the amorphous DLC matrix. A nano-composite can also involve two or more different amorphous matrixes, such as a C—H matrix and separate metal-metal matrix as described in U.S. Pat. No. 5,786,068 to Dorfman et al. In the prior art, these types of films have not been produced with good results with purely PECVD techniques, but only by PVD or hybrid PVD/PECVD methods.
In the case of applications using piping, valves, pumps or tubing for carrying corrosive material, such as the oil/petrochemical industry, the interior surface that is in contact with the corrosive material must be coated. For very low pressure techniques such as PVD, where the pressure is below or near the molecular flow region, coating interior surfaces has been limited to only large diameter and short length (large aspect ratio) tubes. DLCs made using PVD techniques can be produced by sputtering off a graphite target using an Ar plasma. An a-C:H DLC (amorphous hydrogen-containing DLC) can be produced by reactive sputtering by adding a hydrogen background gas. Very high sp3 content DLCs called tetrahedral carbon (ta-C) can be produced using cathodic arc off of a graphite target, due to the very high ionization (˜100%). However, PVD techniques are not practical for coating of internal surfaces, particularly with diameters less than six inches, due to being a line-of-sight process.
Prior art PECVD of DLC based coatings rely on ion bombardment energy to form sp3 bonds. Without this, graphite will form instead of diamond. It has been found that approximately 100 eV of energy on the C+ ion is needed to maximize the sp3 content. This carbon ion energy is a function of bias voltage, pressure, precursor gas and plasma density. High plasma density, low pressure (<1e-3 torr) PECVD techniques such as ECR (electron cyclotron resonance) have generated the highest sp3 content PECVD films, with reports of up to 70% sp3 content. Because these processes are limited to low pressure the deposition rate is very slow (˜1 μm/hr). Prior art precursors are hydrocarbons, such as methane, acetylene and benzene. The precursor used to form the film will change the energy per carbon atom due to the breakup of the molecule on impact with the surface. Thus, a carbon atom produced from acetylene (C2H2) will have approximately one-half the energy of a carbon atom from methane (CH4). Therefore, a higher bias voltage is required to produce high sp3 content films if large precursor molecules are used. The use of a large precursor molecule can have negative effects, such as a larger thermal spike, which causes the sp3 bond to relax back to graphite or sp2. The formation of prior art DLC films is fully described in “Diamond-Like amorphous carbon,” J. Robertson, Materials Science and Engineering R 37 (2002) pages 129-281; incorporated herein by reference. The commonly accepted model of DLC formation is referred to as the ‘subplantation’ model. This model states that for hydrocarbon precursors if the carbon atom arrives with a low energy (˜<50 eV) it will form a high hydrogen content polymer, and if it arrives with moderate energy (˜70 eV-120 eV) it will penetrate below the surface where it is held in compression and forms a sp3 or tetrahedral bond, if the energy is increased further a ‘thermal spike’ will occur locally which allows the sp3 bond to relax back to graphite or sp2 bonding. These numbers are approximate for low pressure and CH4 precursor, and will vary based on pressure and precursor size.
Plasma-enhanced CVD (PECVD) allows coating with reduced temperature, for a temperature-sensitive substrate, by supplying energy from the plasma instead of heat. The invention described herein relates to the PECVD technique, although it is also applicable to PVD processes. PECVD-produced DLCs will contain some hydrogen due to the use of hydrocarbon precursors. In comparison, DLCs made using PVD techniques have less hydrogen. However, as previously mentioned, PVD techniques are not practical for the coating of internal surfaces, particularly with diameters less than six inches. Higher pressure (>10 m Torr) PECVD techniques have the advantage of higher deposition rates, however with prior art techniques it is not possible to make high sp3 content films due to the lack of a collision-less plasma sheath (the mean free path of the ion is less then the plasma sheath width) resulting in low ion energy, additionally the ion/radical ratio is lower at higher pressure. For high quality DLC's it is important to have a large portion of film deposition due to ion flux vs. non-ionized flux, due to the importance of ion bombardment energy. A high level of radicals vs. ions is detrimental to DLC properties, as radicals are highly reactive but lack the energy of ions. Since the ion/radical ratio decreases with increasing pressure prior art process were limited to low pressure for high sp3 content films and were limited to the resulting low deposition rates.
Prior art PECVD techniques contained substantial amounts of hydrogen due to the hydrogen contained in the hydrocarbon precursor which is incorporated into the DLC. This hydrogen has detrimental effects such as lowering the hardness and temperature stability of the coating.
The plasma immersion ion implantation and deposition (PIIID) technique has been shown to be useful for coating the external surfaces of complex shapes. PIIID is performed by applying a negative bias to the workpiece, which will pull positive ions toward the workpiece, if the plasma sheath is conformal. There are also improvements that can be made to film properties such as adhesion and film density via ion bombardment of the workpiece. Use has been made of high sp3 seed material in prior art PECVD formation of carbon-coated barrier films. For example EP 0763 144 B1 uses a diamondoid precursor at very low concentration (<10%) compared to the concentration of a standard hydrocarbon precursor such as acetylene. Internal DLC coatings are not commonly done in the prior art. The deposition of external DLC coatings is well described in Massler (U.S. Pat. No. 6,740,393), this coating description includes an adhesion layer, gradient layer and DLC top coating. One of the advantages taught by Massler is a high deposition rate process preferably in the range from 1-4 microns/hour at a pressure from 10−3 to 10−2 mbar (0.75-7.5 m Torr), the maximum hardness given in the examples taught by Massler is 2,500 HK. In comparison the present invention achieves a much higher deposition rate with high hardness and a higher operational pressure. However in the prior art the ability to control film properties is limited by both the low concentration of diamondoid and the inability to control ion bombardment energy and was limited in application to gas permeation barriers. A comparison of prior art (Massler) and the present invention process parameters are shown below:
InventionMassler(Example F)Process Parameters(Example 2)INTERNALPressure (mtorr)0.75-7.5300Argon flow (sccm)5090Acetylene flow (sccm)350Adamantane flow (liquid ccm)00.2Voltage (V)700800Power (DC Watts)215MagnetsYesNoDeposition rate (μm/hr)1.521.50Hardness (GPa)2524.2
The above is an example of the process and does not limit the range of the invention, for example the process can be optimized to provide a higher hardness then the above at a somewhat lower deposition rate or it can be optimized to provide a high deposition rate with a lower hardness.
Higher pressure (>10 mTorr) PECVD techniques have the advantage of higher deposition rates, however with prior art techniques it is not possible to make high sp3 content films due to the lack of a collision-less plasma sheath. This means that the mean free path of the ion is less than that of the plasma sheath width, resulting in low ion energy. Additionally, the ratio of (free) radicals to ions is higher at high pressure which results in sp2 rich films. A high level of radicals vs. ions is detrimental to DLC properties, as radicals are highly reactive but lack the energy of ions. To form high quality DLC it is important to have a large portion of film deposition due to ion flux vs. non-ionized (or radical) flux, due to the importance of ion bombardment energy. Since the ion/radical ratio decreases with increasing pressure, prior art processes for sp3 formation were limited to low pressure, and the resulting low deposition rates that go along with low pressure.
There is a trend in increasing hardness with increasing saturation, or sp3 bonding, of the precursor molecule. This is because molecules such as acetylene with two pi bonds are more likely to form reactive radicals then a molecule such as methane with sp3 bonding or no pi bonds. Thus a higher hardness film is produced by methane then acetylene, conversely due to the higher radical reactivity the acetylene based coating will have a higher deposition rate then the methane based coating.
Most prior art precursors are hydrocarbons such as methane, acetylene and benzene. The precursor used to form the film will change the carbon energy due to the breakup of the molecule on impact with the surface. Thus a carbon atom produced from acetylene (C2H2) will have approximately one-half the energy of a carbon atom from methane (CH4). Therefore a high bias voltage is normally required to produce high sp3 content films when larger precursor molecules are used. The use of a large hydrocarbon precursor can also have negative effects, such as a large thermal spike.
Prior art PECVD techniques contained substantial amounts of hydrogen due to the hydrogen contained in the hydrocarbon precursor which is incorporated into the DLC. This hydrogen has detrimental effects such as lowering the hardness and temperature stability of the coating.
Compared to CVD techniques, PECVD allows coating at lower temperature because the energy is supplied by the plasma rather than heat. This is important in the instance where the substrate is temperature-sensitive.
Plasma immersion ion implantation and deposition (PIID) techniques have been shown to be useful for coating the external surfaces of complex shapes. PIID is performed by applying a negative bias to a workpiece, and this bias will pull positive ions toward the workpiece if the plasma sheath is conformal. There are also improvements that can be made to film properties such as adhesion and film density via ion bombardment of the workpiece.
Use has been made of high sp3 seed material in prior art PECVD formation of carbon-coated O2 barrier films on plastic materials. For example, EP 0763 144 B1 uses a diamondoid precursor at very low concentration (<10%) compared to the concentration of a standard hydrocarbon precursor such as acetylene. In the prior art, however, the ability to control film properties is limited by both the low concentration of diamondoid and the inability to control ion bombardment energy.
Diamondoids of the adamantane series are hydrocarbons composed of fused cyclohexane rings which form interlocking cage structures that are very stable. The lower diamondoids have chemical formulas of C4n+6H4n+12 where n is the number of cage structures. A complete description of these materials can be found in “Isolation and Structure of Higher Diamondoids, Nanometer-Sized Diamond Molecules” (Dahl, Liu & Carlson, Science, Jan. 2003, Vol. 299), which is incorporated herein by reference. The first three unsubstituted diamondoids are adamantane, diamantane and triamantane.
The term “diamondoids” refers to substituted and unsubstituted caged compounds of the adamantane series including adamantane, diamantane, triamantane, tetramantane, pentamantane, hexamantane, heptamantane, octamantane, nonamantane, decamantane, undecamantane, and the like, including all isomers and stereoisomers thereof. The compounds have a “diamondoid” topology, which means their carbon atom arrangement is superimposable on a fragment of an FCC diamond lattice. Substituted diamondoids comprise from 1 to 10 and preferably 1 to 4 independently-selected alkyl substituents. Diamondoids include “lower diamondoids” and “higher diamondoids.
The term “lower diamondoids refers to adamantane, diamantane and triamantane and any and/or all unsubstituted and substituted derivatives of adamantane, diamantane and triamantane. These unsubstituted lower diamondoid components show no isomers or chirality and are readily synthesized, distinguishing them from “higher diamondoids.”
The term “higher diamondoids” refers to any and/or all substituted and unsubstituted tetramantane components; to any and/or all substituted and unsubstituted pentamantane components; to any and/or all substituted and unsubstituted hexamantane components; to any and/or all substituted and unsubstituted heptamantane components; to any and/or all substituted and unsubstituted octamantane components; to any and/or all substituted and unsubstituted nonamantane components; to any and/or all substituted and unsubstituted decamantane components; to any and/or all substituted and unsubstituted undecamantane components; as well as mixtures of the above and isomers and stereoisomers of tetramantane, pentamantane, hexamantane, heptamantane, octamantane, nonamantane, decamantane, and undecamantane. Adamantane chemistry has been reviewed by Fort, Jr. et al. in “Adamantane: Consequences of the Diamondoid Structure,” Chem. Rev. vol. 64, pp. 277-300 (1964). Adamantane is the smallest member of the diamondoid series and may be thought of as a single cage crystalline subunit. Diamantane contains two subunits, triamantane three, tetramantane four, and so on. While there is only one isomeric form of adamantane, diamantane, and triamantane, there are four different isomers of tetramantane (two of which represent an enantiomeric pair), i.e., four different possible ways of arranging the four adamantane subunits. The number of possible isomers increases non-linearly with each higher member of the diamondoid series, pentamantane, hexamantane, heptamantane, octamantane, nonamantane, decamantane, etc.
Adamantane, which is commercially available, has been studied extensively. The studies have been directed toward a number of areas, such as thermodynamic stability, functionalization, and the properties of adamantane-containing materials. For instance, the following patents discuss materials comprising adamantane subunits: U.S. Pat. No. 3,457,318 teaches the preparation of polymers from alkenyl adamantanes; U.S. Pat. No. 3,832,332 teaches a polyamide polymer forms from alkyladamantane diamine; U.S. Pat. No. 5,017,734 discusses the formation of thermally stable resins from adamantane derivatives; and U.S. Pat. No. 6,235,851 reports the synthesis and polymerization of a variety of adamantane derivatives. The use of lower diamondoid moieties in conventional polymers is known to impart superior thermal stability and mechanical properties.
The coating of internal pipe surfaces has previously been disclosed in U.S Patent Application Pub. No. 20060011468, the method involves using the pipe itself as a vacuum chamber, coupling the gas supply to one opening and the vacuum pump to another, a voltage biasing system is connected with the negative terminal attached to the pipe and with return anode(s) located at the ends of the pipe. Hydrocarbon precursors can be introduced and the voltage biasing system is used to generate a high density hollow cathode plasma and attract hydrocarbon ions to the surface to from a DLC film.